1. Field of Invention
This invention relates to systems and methods for the removal of residual ammonia from flue gas of a power plant that have been subjected to selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) of oxides of nitrogen (NOx), and more specifically, to a process that removes excess ammonia from flue gas by chemical oxidation.
2. Description of Related Art
In recent years, there has been increasing public and government concern over the environmental impacts of the emissions of power plants. For example, the exhaust gas of coal-fired power plants contains pollutants such as nitrogen oxides (“NOx”) and sulfur oxides (“SOx”), as well as particulates termed “fly ash”. Environmental laws establish permissible levels of gaseous pollutants and particulates that may be emitted from the exhaust stack of the plant. In order to reduce the levels of NOx emissions from power plants, as required by environmental regulations, many electric generating units are forced to remove NOx from the flue gas. Various types of pollution control equipment are available to reduce the levels of gaseous pollutants and particulates from the flue gas before it reaches the exhaust stack.
In one typical way of removing NOx from the flue gas, a nitrogenous compound, such as ammonia or a urea based reagent, is injected into the flue gas stream. The ammonia reacts with the NOx to form nitrogen gas and water, thus reducing the NOx content of the flue gas. The reaction of ammonia and NOx may be performed at high temperature without a catalyst, a process termed “selective non-catalytic reduction” (SNCR), or at lower temperature in the presence of a catalyst, a process termed “selective catalytic reduction” (SCR). SNCR is accomplished by injecting ammonia or urea based reagents into the upper furnace to reduce the oxides of nitrogen without the use of a catalyst and permitting the reduction reaction to occur in the flue gas. SNCR reactors typically operates at flue gas temperatures ranging between 850° C. and 1150° C. SCR is generally accomplished at lower temperatures than SNCR, and necessitates the use of a catalyst, such as vanadium oxide, which is placed onto surfaces of catalyst modules positioned within the flue gas stream where the ammonia reacts to reduce the oxides of nitrogen. SCR reactors typically operates at flue gas temperatures ranging between 300° C. and 450° C. At coal-fired power plants, ammonia injection systems for SCR and SNCR reactors are typically installed in the high-temperature and high-dust region of the flue gas stream that is typically located prior to combustion air pre-heaters and ash collection.
It is important to accomplish the reaction of the ammonia and NOx in an efficient manner for maximum possible reaction of the NOx. For selective catalytic reduction (SCR) of oxides of nitrogen with ammonia to work well and result in the lowest values of NOx, it is preferable to be able to use excess ammonia. However, when the quantity of ammonia used is high enough to effectively remove the NOx through SCR, some of the excess ammonia will go through the catalyst unchanged and exit as “ammonia slip” in the flue gas. Ammonia slip may cause downstream equipment problems such as clogging of the space between adjacent air preheater heating elements because of the formation of ammonium sulfate/bisulfate, and/or agglomerated fly ash. The ammonia slip problem is further exacerbated as the result of SCR catalyst surface deterioration as well as misdistribution in flue gas velocity, temperature, and concentrations of ammonia and NOx.
Another major problem created by ammonia slip in coal fired plants is the ammonia contaminates the fly ash. Many power plants dispose of the collected fly ash by selling it to purchasers who further process the fly ash for commercial uses such as for use in mixtures with cement to make concrete. The degree of ammonia contamination in the fly ash, and associated concentration levels, vary among power plants depending on the rate of ammonia injection, the performance of the SCR or SNCR process, the amount of SO3 in the flue gas and the associated operating conditions of the boiler and air pollution control devices. It has been observed that fly ash produced from high sulfur eastern bituminous coal (Class F fly ash) adsorbs more ammonia than fly ash produced from low sulfur western sub-bituminous coal (Class C fly ash). The presence of sulfur in the flue gas increases the associated deposition of ammonia in the form of (NH4)2SO4 and NH4HSO4. The high alkaline condition of Class C ash inhibits ammonia cation (NH4+) formation.
Typical ammonia concentrations on fly ash, as a result of ammonia injection, ranges between 50-120 mg/kg for SCR generated fly ash, 250-600 mg/kg for SNCR generated fly ash, and 700-1200 mg/kg for ESP generated fly ash. When ammonia-laden fly ash is used in cementitious slurry applications, the ammonium salts dissolve in water to form ammoniam cations (NH4+). Under the high pH (e.g., pH>12) condition created by cementitious alkali, ammonium cations (NH4+) are converted to dissolved ammonia gas (NH3). Ammonia gas evolves from the fresh cementitious slurry into the air, exposing workers. The rate of ammonia gas evolution depends on ammonia concentration, mixing intensity, exposed surface, and ambient temperature. Ammonia has no measurable effect on concrete quality (strength, permeability, etc.).
Ammonia gas odors could range from mildly unpleasant to a potential health hazard. Ammonia odors are detected by the human nose at 5 to 10 ppm levels. The OSHA threshold and permissible limits are set at 25 and 35 ppm for the time weighted average—eight-hour (TWA 8-hr) and the short term exposure limit—fifteen-minute (STEL 15-min), respectively. Ammonia gas concentration of 150-200 ppm can create a general discomfort. At concentrations between 400 and 700 ppm ammonia gas can cause pronounced irritation. At 500 ppm, and above, ammonia gas is immediately dangerous to health; at 2,000 ppm, death can occur within minutes. Other than OSHA exposure limits, there are no regulatory, industry or ASTM standards or guidelines for acceptable levels of ammonia in fly ash. However, based on industry experience, fly ash with ammonia concentration at less than 100 mg/kg does not appear to produce a noticeable odor in ready-mix concrete. Depending on site and weather conditions, fly ash with ammonia concentration ranging between 100-200 mg/kg could result in unpleasant or unsafe concrete placement and finishing work environment. Fly ash with ammonia concentration exceeding 200 mg/kg produces unacceptable odor when used in ready-mixed concrete applications. If the ammonia amount of which adheres to the fly ash is relatively high there can be potential health risks from ammonia gas when mixing the fly ash with the cement slurry. In addition to the risk of human exposure to ammonia gas evolving from concrete produced using ammonia laden ash, the disposal of ammonia laden ash in landfills and ponds at coal burning power stations could also create potential risks to the environment. Upon contact with water, the ammonium salts leach into the water and could be carried to ground water and nearby rivers and streams causing potential environmental damage such as ground water contamination, fish kill and eutrophication. Ammonia gas could also evolve upon wetting of alkaline fly ashes, such as those generated from the combustion of western sub-bituminous coal. Water conditioning and wet disposal of alkaline fly ashes would expose power plant workers to ammonia gas.
The invention herein deals with pollution control systems which utilize ammonia within the process in order to initiate, cause and/or supplement the removal of NOx, and in particular SCR, SNCR and/or staged systems (i.e., systems which include one or more SCR or SNCR reactors). The process to be described herein uses a chemical injection system to reduce the ammonia slip by reacting the ammonia with an oxidizing agent.